The present invention relates to an electrostatic printer and in particular to a non-impact electrostatic printer which uses an OFT unit.
An OFT unit in a non-impact electrostatic printer is a device for transmitting an optical image to a photosensitive drum to form an electrostatic image. The OFT unit comprises a cathode ray tube and an optical fiber plate or head plate. This head plate, which is approximately 5 mm thick, is composed of a bundle of glass fibers normally having a diameter of approximately 20 .mu.m. The head plate is joined to the surface of the cathode ray tube and it is located a slight distance of 0.1 to 0.2 mm away from the surface of the photosensitive member. Because a slight amount of static electricity is formed on the head plate, toner particles are attracted to the surface of the head plate to contaminate the head plate. The toner which adheres to the head plate prevents the formation of a clear image on the photosensitive member.
Different techniques have been employed in the prior art to prevent the surface of a head plate from being contaminated with toner particles. For example, Japanese Patent Disclosure No. 49-125038 issued on Nov. 29, 1974 and Japanese Patent Disclosure No. 53-118534 issued on Sept. 20, 1978 disclose a transparent film between the surface of the photosensitive drum and the head plate. The transparent film is moved relative to the photosensitive drum. In addition, in the above Japanese Patent Disclosure No. 49-125038, a cleaning liquid is applied between the film and the photosensitive drum. However, since the cleaning liquid must flow between the photosensitive drum and the film, the location of the OFT unit is greatly restricted and the photosensitive drum must have a greater diameter. The apparatus in this latter patent disclosure also requires a cleaning liquid dispenser, a cleaning liquid collector and a voltage application device for applying a voltage to the cleaning liquid. As a result, the apparatus is large, complicated and cumbersome. On the other hand, in Japanese Patent Disclosure No. 53-118534, cleaning plates are required for cleaning the transparent film. These cleaning plates suffer several of the same disadvantages discussed above with respect to the cleaning liquid of the apparatus in Japanese Patent Disclosure No. 49-125038.
In the apparatus described in both the above Japanese patent disclosures, the transparent film is subject to sagging or elongation if a certain period of time expires before the next printing cycle of the electrostatic printer. Also, the film in these disclosures touches the surface of the photosensitive drum which causes contamination of the film.